THE APPEARANCE OF FORESTS

The evolution of leaves, lateral meristems, and secondary tissues (wood & bark) revolutionized the growth opportunities for Devonian plants. These advances are best developed in the Progymnosperms. This Class first appears in the Givetian (Upper Middle Devonian) and becomes extinct in the Early Carboniferous. It is believed that the Trimerophytes were the stock ancestors.

Three groups of progymnosperms coexisted in the Middle and Late Devonian. They possessed pycnoxylic wood and reproduced either by homospory or heterospory. Members of these plant groups comprised the Earliest Forests. These groups include:

ANEUROPHYTALES

These plants are known from the Eifalian to Frasnian, and may represent a variety of growth habits. Branching patterns in the laterals were three dimensional, but the ultimate branches (functional leaves) may be helical, decussate, or planar in arrangement. Maturation of primary xylem is mesarch, and wood tissues completely surround the central xylem core. Eospermatopteris is a Middle Devonian tree attaining heights of 9-12 meters. This is based on the association of these trees with compression/permineralized assemblages in the same strata.

ARCHAEOPTERIDALES

These trees (Callixylon specimens measuring 1.53 meters in diameter and >8.6 meters in length) were responsible for one of the first forestation in the Givetian. The pycnoxylic wood is characterized by radial walls with circular bordered pitting in addition to the tangential wall pitting. True leaves appear on the branches of this group and consist of webbed (laminate) structures that may be simply divided or entire. Branches are variously arranged in the plant group. Plants in this group are either homosporous or heterosporous (4 taxa identified, to date). Heterosporous microspores range from 30-70 m, whereas megaspores range from 150-500 m in diameter.

PROTOPITYALES

These plants were erect shrubs/trees with stems greater than 0.45 m in diameter is known from the Early Carboniferous. The stem displays endarch maturation, and the wood is composed of tracheids with circular bordered pits. Reproduction was fern-like with the development of spores. Most specimens are homosporous, although two size classes of spores have been identified and heterospory can't be ruled out.


MIDDLE TO LATE DEVONIAN COMMUNITY STRUCTURE

The transition from the Late Early Devonian low-canopied (< 2meters) densely packed rhizomatous assemblages to the innovations of the Middle Devonian was due to independent evolution of rooting systems, arborescence, and wood in several major groups. Our knowledge of Early Devonian systems is based mostly on wetlands where vegetative propagation predominates in an attempt to occupy available space. The system was probably controlled largely by patch dynamics and colonization patterns.

The consequences of the Middle Devonian innovations includes the development of true stratification and tiering with canopies attaining heights of > 10 meters. Plants also were capable of inhabiting drier conditions (periodic drying) and there was probably a good understory (sphenophytes [horsetails] and progymnosperms) and ground cover (e.g. the lycophyte Leclerquia). With the advent of the Archaeopteridales, trees become more common.

Few studies have attempted to emphasize the community relationships in the Devonian. It has been recognized that different floral assemblages occur in different sedimentological facies. In the Givetian of New York State Matten (1974) notes that although Aneurophytalean progymnosperms occur in various localities, the subordinate floral elements differ. Floras preserved in thin, fine-grained black shales are comprised of Rhacophyton (a putative early fern) and cladoxylalean ferns coexist with the aneurophytes (Cairo), whereas in coarser-grained shales/siltstones (Gilboa) lycophytes and sphenophytes are preserved along with the aneurophytaleans.

The Late Frasnian is a time of terrestrial extinctions with entire clades eliminated including most herbaceous lycophytes and anuerophytalean progymnosperms. Low diversity assemblages are preserved during this interval including arborescent forms of Archaeopteris (k-selective) and Leptophloem (a lycophyte). The advent of seed habit and seed-bearing plants in the Famennian begins to increase community diversity. Floras collected at this time reflect r-selective strategies, but also the environments of deposition from which they are collected reflect this taphonomic bias.


LATE DEVONIAN BIOTIC CRISIS

Givetian - Fammenian important paleobotanical developments:

In the Frasnian-Fammenian crisis, 21% families and 50% genera marine organisms extinct. It is an unusual event in that:

Late Devonian shows major excursions in sedimentological and chemical records (e.g., Black Shales; organic-rich shales and coals (sequester light Carbon enrichment in marine 13C by 4%. Increased Carbon burial and enhanced silicate weathering reduced pCO2 from 12-16 PAL (Present Atmospheric Level) to 1 PAL in the Carboniferous.

The arrborescent & seed habit allowed plants to adapt to diverse ecological conditions and occupy drier extrabasinal environments.

Middle-Late Devonian anoxia parallels development of woody arborescents and maximum size. Rapid spread of Archaeopteris in Late Devonian is at time of Kellwasser event. Hagenberg event (Devonian-Carboniferous) is preceeded by development of seeds.

Large increase in root biomass preceded major paleontological, sedimentological, and geochemical events by a few million years. Increase in abundance and biomass. Expanse of global rhizosphere resulting in transient intensification of soil formation and increase in permanently deeply weathered soils. Increase in nutrient flux to the oceans resulting in eutrophication of semi-restricted epicontinental seaways. Algal blooms & Black shales. Drawdown of atmospheric CO2 and global cooling were secondary effects of enhanced silicate weathering and rapid OM burial.


EVOLUTION OF THE SEED HABIT

The GYMNSOSPERMS evolve in the Late Devonian with a change in reproductive habit, the shift from a heterosporous life cycle to that of the seed. The oldest reported seed, Archaeosperma, is 350 MY old and collected from the eastern U.S. It is a cupulate seed. It is found in association with Pteriodspermous elements (foliage, permineralized axes) such as Triphyllopteris and Calamopitys.

The change from a megasporangium to an integumented megasporangium may have occurred by one or more evolutionary modifications. These changes included:

  1. Abortion of three megaspores and formation of a single functional megaspore in the megasporangium;
  2. Retention of the functional and abortive megaspores in the megasporangium;
  3. Formation of an integument that delimited the micropyle;
  4. Formation of a pollen tube or pollen tube-like structure from an endosporic megagametophyte;
  5. Formation of an endosporic megagametophyte within an indehiscent megasporangium (nucellus);
  6. Elaboration of the apex of the indehiscent nucellus for reception of pollen.

NUCELLAR MODIFICATION THEORY:

  1. A multispore megasporangium undergoes reduction in the number of functional megaspores;
  2. The site of development is lowered in the megasporangium towards the base;
  3. The dehiscence site becomes expanded and modified into a primitive micropyle and pollen chamber;
  4. Vascularization of the megasporangium develops such that the conducting tissues grow towards the nucellus;
  5. The megasporangial wall, in essence, becomes the integument.

TELOME THEORY:

  1. Reduction of the sterile and fertile ultimate telomes;
  2. Centralized megasporangium surrounded by sterile telomes branch systems;
  3. Syngenesis of sterile telomes surrounding the megasporangium (becoming the integument); sterile telomic branch systems surround this centralized unit becoming the cupule.

© Copyright 1997 by Robert A. Gastaldo. All rights reserved. No part of these lecture notes may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission from the author.